This invention generally relates to electrical switching circuits and more specifically to bilaterally conducting, wide band, low loss, low distortion electronic switching circuits.
Telephone switching networks incorporate a wide variety of electrical switching elements. Many times these switching elements must be constituted by bilateral, wide band, low loss, low distortion switching elements. Conventionally, these switching elements comprise electromechanical relays and related circuits. When the contacts on a relay close, the resulting metal-to-metal connection forms a conductive path that produces essentially no loss of and no distortion to any signal passing through the contacts and that allows bilateral signal transmission over a wide range of frequencies. The contacts can withstand voltage and current levels that are dependent only on the rating of the contacts themselves. Moreover, these relays provide lightning and other transient signal protection because the transient signals on the conductive path are inherently isolated from the control circuitry that connects to a relay coil.
Unfortunately, such electromechanical devices have a number of known characteristics that are detrimental in many applications including telephony applications. Life expectancy of a relay depends upon contact wear. As contact wear is not predictable on an accurate basis, it is difficult to predict relay life. Moreover, electromechanical relays tend to be expensive, and this expense is subject to high cost multiples in telephone switching networks where one such relay can be used for each of the many telephone lines connected to the telephone switching network. Electromechanical relays also increase other manufacturing costs as they are discrete elements and must be mounted on circuit boards individually.
Other special switching circuits have been proposed from time to time to replace the electromechanical relay. Reversely poled, silicon-controlled rectifier and other transistor circuits have been suggested. However, in many telephone applications analog voice signals pass through such a switching circuit and the reversely poled transistor and SCR circuits introduce a crossover distortion as these circuits tend to stop conducting near the zero voltage, or current crossover, point. Circuitry can be added to compensate such distortion. However, this and other circuitry that may be necessary to provide proper operation increase the overall costs of manufacture.
More recently, a gated diode switch has been proposed that can be manufactured with large scale integration manufacturing techniques. The large number of such switches in telephone switching networks makes the use of large scale integration manufacturing techniques highly desirable, especially in view of the attendant cost savings that can be achieved. The proposed switch, however, has certain disadvantages. First, this circuit requires high voltages in the orders of up to 600 volts. When the switch is located in the standard tip and ring conductors to and from a subscriber's handset, certain precautions must be taken to protect customers and telephone company personnel against inadvertent contact with these voltages. This switching circuit also requires three separate voltages and this, of course, introduces complexities in the power supplies that serve the telephone switching network. Moreover, it would appear from available information that the proposed switch may require compensation for crossover distortion when the switching circuit conveys analog voice signals.